1. Field of the Invention
The present invention is generally concerned with the calibration that has to be carried out when starting up an ophthalmic lens grinding machine in order to determine with all the required accuracy, relative to its general reference system of axes, various parameters relating to the machining tools that can be used therein, and in particular the position of a machining tool in the reference system of axes, given that the calibration can, and even must, be repeated periodically, in particular when changing or sharpening the machining tool.
The present invention is more particularly directed to the situation in which the grinding machine to be calibrated includes a swing-arm which pivots on a frame, a lens-holder shaft which rotates on the swing-arm about a rotation axis parallel to the pivot axis of the swing-arm, and a tool-holder shaft which is carried by the frame at a distance from the pivot axis of the swing-arm and on which a machining tool can be mounted.
2. Description of the Prior Art
U.S. Pat. No. 5,806,198 proposes substituting a calibration template for a lens on the lens-holder shaft in order to carry out the required calibration, performing an approach phase during which the swing-arm, equipped in this way with a calibration template of this kind, is moved toward the tool-holder shaft, which is equipped with the machining tool whose co-ordinates are to be determined, and having interruption of the approach phase commanded by a sensor provided for this purpose and responsive to contact between the swing-arm and a component, usually referred to as the drive link, which during the machining of a lens drives the swing-arm in accordance with the machining to be effected, providing a support for the swing-arm when sufficient material has been removed.
The above arrangements have proved satisfactory and may continue to do so.
They nevertheless have the following drawbacks.
Firstly, detection occurs at a distance from the lens-holder shaft and from the tool-holder shaft with a lever arm that is less than that of the two shafts, which is detrimental to the accuracy of the results obtained.
Also, and most importantly, once detection has occurred the full weight of the swing-arm rests on the machining tool.
Although some machining tools are able to support a weight of this magnitude without deformation, and this is in fact the case for a grinding wheel, it is not necessarily the case for all possible machining tools.
To be more precise, it has now been proposed to use a relatively fragile machining tool on at least some grinding machines, namely a grooving tool to be used when it is necessary to machine a groove into the edge of a lens.
This is the case in particular with lenses to be mounted in spectacle frames, like those sold under the tradename "Nylor", in which each circle or surround includes a rigid part and a filament which extends from one end of the rigid part to the other.
The grooving tool is usually on a secondary tool-holder shaft separate from and parallel to the main tool-holder shaft carrying one or more grinding wheels, the secondary tool-holder shaft being a simple spindle, for example, extending cantilever fashion from a retractable dedicated frame.
The grooving tool is usually a simple, relatively thin and narrow grinding wheel.
During the approach phase of the calibration process, the weight of the swing-arm resting on a grooving wheel of the above kind would inevitably cause unwanted and unacceptable deformation of the grooving wheel.
A general object of the present invention is an arrangement which avoids the above drawbacks and which in particular prevents deformation of a grooving wheel when calibrating it.